Ion-Sensitive Electrodes

The present invention relates to ion-sensitive electrodes for determining the concentrations of certain mainly cations in electrolytes.

Known ion-senstive electrodes for monovalent cations have several disadvantages; erosion by electrolytes in which the electrodes are placed tends to occur, the maximum temperature at which such electrodes can be used is the boiling point of water and since the electrodes often take the form of glass electrodes with an internal electrolyte they are relatively fragile. In addition known ion-sensitive electrodes sensitive to monovalent cations tend to be unstable with time and temperature. Further the concentrations of certain monovalent cations such as Rb.sup.+, Cs.sup.+ and Li.sup.+ could not formerly be measured at all by electro-analytical techniques using ion-sensitive electrodes. It is expected that at least some of the above disadvantages will be wholly or partly overcome by the present invention.

According to a first aspect of the present invention there is provided an ion-sensitive electrode for determining the concentration of selected cations other than proton in at least one electrolyte, comprising a member which includes a material of non-stoichiometric composition, having a high ionic mobility, containing cations of the type whose concentration is to be determined, and having a structure which allows interactions requiring electrons and cations of the said type to take place at the electrode surface and/or within the material, and means for making electrical contact with the member.

The said member may have a crystal lattice structure with a spatial configuration which allows ion transfer to take place and also gives the material a high ionic mobility.

According to a second aspect of the present invention there is provided an ion-sensitive electrode for determining the concentration of selected cations in at least one electrolyte, comprising a first member which includes a first material having a high ionic mobility and which contains cations of the type whose concentration is to be determined, the first material not being capable of substantial chemical reaction with at least one electrolyte, a second member in contact with the first member, which includes an electrically conductive second material which also contains cations of the said type and which allows interactions requiring the supply of electrons and the said cations to take place internally, and/or at the interface between the first and second members, and means for making electrical contact with the second material, the electrode being so constructed that the second member, but not the first is isolated in operation from electrolytes.

Electrodes according to the second aspect of the invention are primarily for use where the second material reacts chemically with the said electrolyte.

Cations whose concentration may be determined by using electrodes in accordance with the first or second aspects of the invention include: Na.sup.+, K.sup.+, Rb.sup.+, Li.sup.+, Tl.sup.+, Ag.sup.+, Cu.sup.+, NO.sup.+, Cs.sup.+, and NH.sub.4.sup.+. Thus it can be seen that a wide range of ion-sensitive electrodes according to the invention is available and this range includes electrodes for some ions which previously could not be measured by electro-analytical techniques.

In electrodes in accordance with the first aspect of the invention the non-stoichiometric material may be a tungsten bronze such as sodium tungsten bronze, Na.sub.x WO.sub.3 where x varies from just above 0 to 1, or another inorganic bronze which has a spatial configuration allowing transfer of the ion of interest; for example a titanium, vanadium or molybdenum bronze.

The second material of the second aspect of the invention may also be a non-stoichiometric material such as one of the bronzes mentioned above.

The Na.sup.+ cation in sodium tungsten bronze may be replaced wholly or partly with K.sup.+, Li.sup.+, and NH.sub.4.sup.+ and the resulting material (Na.sub.x M.sub.y WO.sub.3 where M represents K.sup.+ or Li.sup.+ or NH.sub.4.sup.+ and y varies from just above 0 to 1 and x plus y must not be greater than 1) then behaves reversably to the substituted cation and may be used as the non-stoichiometric material in electrodes according to the first and second aspects of the invention. Methods of preparing the above-mentioned tungsten bronzes are described in "Fast Ion Transport in Solids" edited by Van Gool (1973) published by North Holland, and such methods may also be found in the Quarterly Review, 22, 30 (1968).

In the second aspect of the invention the first material may be a .beta. alumina containing cations of the said type and the second material may be a mercury amalgam also containing cations of the selected type.

A common .beta. alumina is sodium .beta. alumina whose formula is nominally given as Na Al.sub.11 O.sub.17. Substitution for cations other than Na.sup.+ can readily be achieved for Li.sup.+, K.sup.+, Rb.sup.+, Ag.sup.+, Tl.sup.+, NH.sub.4.sup.+ and In.sup.+ and is described in the Journal of Inorganic Nuclear Chemistry 29, 2453 (1967). Similar substitutions are described for NO.sup.+ in Inorganic Chemistry 8, 2531 (1969); for Ga.sup.+ in Inorganic Chemistry 8,994 (1969); and for Cu.sup.+ in U.S. Government Research and Development Report 16 No; AD 693,158, (1969) M. S. Whittingham, R. W. Helliwell and R. A. Huggins.

It will therefore be seen that ion-sensitive electrodes according to the invention may be constructed from solid materials and for this reason are less fragile than many known electrodes which have internal electrolytes and can be used at much higher temperatures, at least up to 200.degree. C. The specified materials are more chemically durable than those used in many known ion-sensitive electrodes. Since electrodes according to the invention may be constructed with a relatively low electrical resistance, they are easily used with conventional potential measuring equipment for ion-sensitive electrodes.

In a preferred ion-sensitive electrode according to the second aspect of the invention the first member includes .beta. alumina, the second member includes solid mercury amalgam, the means for making electrical contact with the second material includes a platinum contact means, and both the .beta. alumina and mercury amalgam contain cations of the type which it is desired to detect.

In a further preferred electrode according to the second aspect of the invention the first member includes .beta. alumina, and the second member includes a tungsten bronze.

Of the two preferred electrodes the former is useful in aqueous electrolytes which would react with the mercury amalgam if it were not for the .beta. alumina member; and the second preferred electrode is useful for media which might attack tungsten bronze.

Certain embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a cross-section through a first embodiment of the invention including a .beta. alumina member protecting a mercury amalgam member.

FIG. 2 is a cross-section through part of an ion-sensitive electrode having a .beta. alumina member protecting a tungsten bronze member,

FIG. 3 shows a cross-section through a third electrode according to the invention which uses a tungsten bronze member as the ion-sensitive member, and

FIG. 4 shows apparatus for determining the concentration of selected ions using the ion sensitive electrode of FIG. 1.

In FIG. 1 a sodium .beta. alumina disc 10 is held in contact with sodium-mercury amalgam 11 by a surrounding casing of araldite 12. The amalgam is also in contact with a platinum disc 13 fixed by araldite to a brass nut 14 in electrical contact with the platinum. When the araldite 12 has been cured, a length of brass studding 15 is screwed to the nut 14 and also to another nut 15 which holds the electrode assembly to the PTFE body 17 and acts as the electrical connection to the electrode. At the same time additional araldite 12' is added to ensure that electrolyte is prevented from reaching the nut 14 or the studding 15 and this additional araldite is then cured.

The solid sodium mercury amalgam is made by drying a small chip of metallic sodium with filter papers and depositing the chip in a small glass vessel which already contains a small amount of dry clean mercury. The glass vessel is shaken and a spontaneous reaction takes place. If, after visible reaction has ceased, the resultant mass is not solid more chips of sodium are added until a solid but pliable amalgam is formed. Other amalgams are made using the same method but substituting a metal whose ions are not to be detected for the sodium.

Before use the electrode is conditioned in a neutral solution of the ion to be detected; for example in this the active part of the electrode is immersed in say 0.1 M NaCl for 24 hours.

In operation, (see FIG. 4) a potential measuring device 25 of the kind conventionally used with ion-sensitive electrodes is connected between the studding 15 and a reference electrode 26, the active part of the electrode of FIG. 1 is immersed in the electrolyte of interest, and this electrolyte is connected in one of the conventional ways to the reference electrode.

The high ionic mobility of the .beta. alumina 10 allows, in this case, sodium ions to travel through the .beta. alumina to the sodium mercury amalgam 11 and the alumina can thus be considered as a solid extension of the electrolyte. The reversible reaction between metallic sodium and sodium ions which takes place at the interface between the .beta. alumina and the amalgam requires electrons and gives rise in the usual way to a potential on the platinum layer 13, the magnitude of this potential depending on the concentration of sodium ions in the electrolyte.

The electrode of FIG. 1 is suitable for the aqueous electrolytes which without the .beta. alumina layer 10 would attack the sodium mercury amalgam. An electrode suitable for use for example in a fused salt electrolyte is shown in FIG. 2. A lithium .beta. alumina layer 18 protects a lithium tungsten bronze member 19. The layer 18 and the member 19 are fixed to a metallic bar 20, for example copper by means of an araldite layer 21. A heat shrunk PTFE sleeve 22 protects the outside of the electrode. As before the .beta. alumina acts a solid extension of the electrolyte and a reversible reaction involving the lithium ions takes place within the tungsten bronze. The high ionic mobility of this material and the lattice defects allow ions to move within the tungsten bronze and alloy ion exchange involving lithium cations and electrons to occur within, or at the surface of the tungsten bronze. Potential measurements giving an indication of ion concentration of lithium are made in the conventional way with the metal bar 20 coupled to one terminal of the potential measuring device which has a reference electrode connected to its other terminal.

The electrode shown in FIG. 3 is for use in an aqueous electrolyte and therefore does not require a protective layer of .beta. alumina. Apart from the absence of this layer construction is similar to that of FIG. 2 with a tungsten bronze member 19 the bar 20 for electrical contact purposes, araldite 21 and a PTFE sleeve 22. The type of tungsten bronze used depends, of course, on the ion to be detected. Suitable tungsten bronzes may be prepared, as mentioned above, when some of these ions wholly or partly replace the sodium in sodium tungsten bronze forming the starting point for these materials.

For electrodes of the type shown in FIGS. 2 and 3 which are sensitive to Na.sup.+ ions, sodium tungsten bronze is used as the tungsten bronze 19. The ionic interaction which then takes place and gives rise to the potentials measured is:

This interaction is possible because Na.sup.+ ions and electrons (e) are able to move freely through the tungsten bronze since its structure provides high ionic mobility.